This invention relates to an optical system for providing an effective image source of deep ultraviolet radiation and more particularly to apparatus for providing a uniform deep ultraviolet radiant source for sub micron resolution photolithography.
In the field of optical exposure systems of the type employed for exposing photoresist material, as deposited on wafers in the production of integrated circuits, the mask may be positioned in close proximity to the wafer at a lens exposure plane. At that plane one desires that the light impinging upon the plane is characterized by a relatively uniform field. As indicated, many such systems exist in the prior art. For examples of suitable prior art systems and explanations of their operation, reference is made to U.S. Pat. No. 4,348,105 entitled "Radiation Shadow Projection Exposure System" issued on Sept. 7, 1982 to Fausto Caprari and assigned to the RCA Corporation. Reference is also made to U.S. Pat. No. 3,860,335 issued on Jan. 14, 1975 entitled "Optical System" to Fausto Caprari and also assigned to the RCA Corporation.
Essentially, the above noted prior art patents describe radiation systems for projecting uniform fields of irradiance to expose a photomask through a transparency in proximity to or in contact with the irradiance sensitive surface, such as a wafer or another mask. The patents describe different light sources which operate in the near and deep ultraviolet wavelength spectrum and various techniques for converting such sources into a suitable pattern which is projected on a plane containing a mask and a photoresist coated wafer.
As one can understand, as integrated circuits become more complex and operate at higher and higher frequencies, one would desire to achieve great resolution in regard to such photoresistive exposure systems whereby the resolution to be achieved will be able to distinguish between printing feature sizes of the order of 1 micrometer or less which are a function of the resist process used.
Thus the prior art has investigated the use of deep ultraviolet (DUV) lithography which employs ultraviolet radiation in the range of 200 to 300 nanometers. Hence this technology has been recognized as a promising economical technology for printing feature sizes on the order of 1 micron or less. Relative to the more exploratory X-ray or E beam technologies, deep UV lithography has the advantage of being able to retain the general configuration of conventional equipment such as the light sources, exposure/alignment optics, mask, wafer and so on, with modifications to use different materials.
Photoresist exposure systems in the prior art employ a pinpoint radiant source, a specular ellipsoidal reflector, Fly eye lens system and collimation lenses or a helical pulse xenon source in a radiation projection optical system which comprises three or four plano convex lenses.
As one will understand, during the past 20 years these systems were constantly improved to meet the demand of the semiconductor industry for higher resolution and better uniformity. Typically these photoresist exposure systems can be used for line/space resolution of 1 micrometer with low yield as a result of non-uniform ray bundles distribution. The use of a Fly eye lens system results in an apparent uniform irradiance level at the wafer plane as desired. This irradiance level, when monitored by a photodetector equipped with a cosine corrector which is insensitive to angular variations of the ray bundles across the wafer, produces such an apparent uniform level at the wafer plane as indicated. In any event, the non-uniformity of the ray bundles regarding the angular distribution across the wafers in these types of systems is the result of asymmetry, non-uniformity, and spatial instability of the radiant source.
In order to implement such requirements for DUV systems, the prior art employs high pressure pinpoint light sources. These sources contain pressurized gas at 5 atmospheres or greater. Certain of these sources employed combinations of mercury and xenon to provide a pinpoint source which are employed in microphotolithography. In any event, such light sources essentially convert 2% of the input energy to energy within the DUV range and hence such sources are relatively inefficient. Such prior art systems dictate the use of high pressure pinpoint sources as the source of illumination which source of illumination is then magnified by an amount of about 3-5 times before impinging upon the mask/wafer plane. Thus, one can understand the type of lamps utilized in these prior art systems have a relatively low conversion efficiency in the desired deep ultraviolet range.
Hence, as indicated above, the non-uniformity of the ray bundles in regard to the angular distribution across the wafer in prior art systems is the result of asymmetry, non-uniformity and spatial instability of the typical prior art radiant sources. As one can further ascertain, tests show that very uniform ray bundle distribution across the wafer is required in order to resolve submicron lines and spaces in the photoresist. Presently available commercial radiant sources are not symmetrical, uniform or spatially stable.
It is therefore an object of the present invention to provide a symmetrical, uniform and spatially stable radiant source for operation in the deep ultraviolet range and which source exhibits superior operating characteristics as compared to prior art sources.